1. Technical Field
The present invention relates to a turbo blower for a laser device for forcibly circulating a laser gas in a gas laser device used for machining and the like and a laser oscillator device, and more particularly, to a turbo blower for a laser device and a laser oscillator device, in which the life of bearings therein is lengthened to improve the reliability and maintenance efficiency of the device.
2. Background Art
Modern carbon dioxide (CO.sub.2) gas laser oscillator devices provide high output and high-quality laser beams, and are now widely used for laser beam machining, such as the cutting of metallic or nonmetallic materials and the welding of metallic materials or the like. The development of these devices has been rapid, especially of CNC (numerical control device) laser machining devices combined with a CNC, in the field of a high-speed, high-accuracy cutting of intricate configurations.
A conventional carbon dioxide (CO.sub.2) gas laser oscillator device will now be described with reference to the drawings.
FIG. 4 is a diagram showing the general arrangement of the prior art carbon dioxide (CO.sub.2) gas laser device. As shown in the Figure, an optical resonator composed of an output coupling mirror 2 and a total reflection mirror 3 is disposed at either end of a discharge tube 1, and metal electrodes 4 and 5 are mounted on the outer periphery of the discharge tube 1. The metal electrode 4 is grounded, while the metal electrode 5 is connected to a high-frequency power supply 6, and a high-frequency voltage from the high-frequency power supply 6 is applied between the metal electrodes 4 and 5, whereby a high-frequency glow discharge occurs in the discharge tube 1 and a laser excitation is effected. Numerals 13 and 14 denote a laser beam axis in the discharge tube 1, and a laser beam axis taken out from the output coupling mirror 2, respectively.
When starting a gas laser oscillator device constructed in this manner, gas in the whole apparatus is first exhausted by a vacuum pump 12 and, then a valve 11 is opened to allow a predetermined amount of laser gas to be introduced from a gas cylinder 10 until the gas pressure in the apparatus reaches a specified value. Thereafter, the exhausting by the vacuum pump 12 and the resupply of gas by the valve 11 are continued, whereby part of the laser gas is continually replaced with fresh gas while the gas pressure in the apparatus is kept at the specified value, and thus gas pollution in the device can be prevented.
As shown in FIG. 4, the laser gas is circulated in the apparatus by a blower 9, for cooling the laser gas. In the carbon dioxide (CO.sub.2) gas laser, about 20% of the injected electrical energy is converted into a laser beam, and the remainder is used for heating the gas. Theoretically, however, the laser oscillation gain is proportional to the minus (3/2)th power of the absolute temperature T, and thus the laser gas must be forcibly cooled in order to raise the oscillation efficiency. In the shown device, the laser gas flows through the discharge tube 1 in the direction indicated by the arrows, at a flow rate of about 100 m/sec, and is introduced into the cooling unit 8, to remove heat, mainly attributable to the electric discharge, from the laser gas. The blower 9 compresses the cooled laser gas, and the compressed laser gas is passed through a cooling unit 7 before being fed into the discharge tube 1. This is necessary in order to remove compression heat produced in the blower 9, by the cooling unit 7, before the gas is again fed into the discharge tube 1. These cooling units 7 and 8 are well known in the art, and thus a detailed description thereof is omitted.
FIG. 5 shows the construction of a turbo blower used as the blower 9. As shown in the Figure, an impeller 16 and a shaft 26 are mechanically connected, and a rotor 17 is mounted on the shaft 26. The rotor 17 and a stator 18 constitute a motor by which the impeller 16 is rotated at a high speed of about 100,000 rpm. In contrast with a low-speed rotation Roots blower, therefore, the reduction of the volume is inversely proportional to the rotational frequency. Further, rolling-contact bearings 19 and 20 are used for supporting the shaft 26, and since the turbo blower rotates at high speed, the rolling-contact bearings 19 and 20 are lubricated by an oil-jet or oil-air lubrication method in such a manner that oil is supplied to the bearings at predetermined intervals.
In an oil supply unit 21 shown in FIG. 5, the oil is atomized by gas and supplied to the rolling-contact bearings 19 and 20 through passages 22 and 23.
With this arrangement, the laser gas is sucked from the cooling unit 8 into the turbo blower, as indicated by an arrow 81, and is discharged from the turbo blower into the cooling unit 7, as indicated by an arrow 71.
The conventional laser oscillator device shown in FIGS. 4 and 5 has the following problems.
First, since oil is used as a lubricant, oil components are mixed with the laser gas, and accordingly, optical parts are polluted and an output drop or mode transformation occurs. In a high-output carbon dioxide (CO.sub.2) gas laser, therefore, the laser gas is continually replaced, which accounts for a large part of the running cost. Nevertheless, the optical parts still must be periodically replaced or cleaned, and thus the maintenance work is labor-consuming.
Second, if too much oil is supplied, the efficiency of the bearings may be lowered due to disturbances in the flow of the oil in the bearings, and further, the temperature thereof is affected. Accordingly, the oil supply unit requires intensive control, which entails a substantial increase in costs.
Third, the bore of the oil passage is very small, and therefore, is easily blocked by clogging, whereby it becomes impossible to supply oil to the bearings, resulting in bearing seizure.
Accordingly, the present inventors previously filed an application (Japanese Patent Application No. 63-148918) in which a grease lubrication method was proposed, to prevent pollution of the laser gas by oil mist. The use of grease for the lubrication ensures that only an irreducible minimum amount of oil escapes, and therefore, optical parts and the like are not polluted by oil. Accordingly, the laser output and beam characteristics will not be deteriorated, and only a predetermined amount of grease need be sealed in the bearings at the time of the assembling thereof. Accordingly, the need for a continual supply of lubricant, and maintenance work, such as oiling, becomes unnecessary.
If the turbo blower is rotated at a high speed (e.g., a DmN value of 800,000 or more), however, the grease is expelled and forced out of the bearings by the centrifugal force created by the rotation of the inner race, retainer, and rolling members thereof, since the amount of grease sealed therein accounts for 30% to 50% of the space inside the bearings. Accordingly, the amount of grease retained between the rolling-contact surface of the inner or outer race and the rolling members is greatly reduced and cannot withstand a prolonged operation, and thus the grease must be periodically replaced or resupplied. During this replacement or resupply, the laser oscillator device must be stopped to allow the turbo blower to be removed and disassembled.
Moreover, when the amount of grease retained becomes too small, the fatigue life of the bearings is shortened due to an increased friction between the rolling-contact surfaces attributable to insufficient lubrication, which friction produces heat, and thus the bearings themselves must be replaced.
In the grease lubrication method, therefore, the amount of grease sealed therein must be strictly controlled. If this is not done, the grease cannot produce the intended lubrication effect, and further, the above problems arise.